It is well known that, in optically non-linear materials, light or other kinds of electromagnetic radiation of certain wavelength can be converted to light or other kinds of electromagnetic radiation of another wavelength. This conversion can be in the form of frequency doubling, sum frequency generation, difference frequency generation, parametric amplification, parametric oscillation, etc., utilizing the so-called second order optical non-linearity. For the conversion to be efficient, so-called phasematching has to be achieved. A problem is, however, that in all homogenous, optically non-linear materials with a uniform structure, the phase-matching condition can only be fulfilled within a rather limited wavelength interval, if it can be fulfilled at all. Another problem associated with conversion is that for conventional non-linear optical materials and methods, the intensity in the light from the pump source has to be high for the efficiency of the frequency conversion to be acceptable.
One way of achieving efficient frequency conversion is to use so-called quasi-phase-matching, thereby making the crystal orientation vary periodically in the optically non-linear material so that the non-linearity changes its sign with the same period. The method implies that when the interacting waves have come 180 degrees out of phase and no longer interact constructively, the crystal structure is altered so that the waves continue to interact constructively.
Crystal structure variation has been realized in the form of plate stacks and in the form of crystals grown in such a way that the crystal orientation alters periodically during the growth. Both methods, however, are complicated and expensive.
Further, it is known that by coupling the light/radiation into a waveguiding layer or channel, one can achieve a high radiation intensity over a long interaction length, thereby reducing the demands on the intenstity of the pump source. This means that low power lasers can be used for frequency conversion. Examples of materials in which waveguides with low optical losses can be fabricated are LiNbO.sub.3, LiTaO.sub.3 and KTiOPO.sub.4.